Signals from satellite-based positioning systems, such as the Global Positioning System (GPS), are often used to determine locations of mobile devices. GPS signals, however, are not always received reliably, such as when the receiver is located inside a building. In addition, the accuracy of GPS location fixes for many receivers is no better than about 10 meters.
As an alternative to GPS, wireless signals transmitted by terrestrial signal sources can be used to determined locations of mobile devices. In one approach, a terrestrial signal source transmits a wireless signal that includes an indication of its transmission time. A mobile device receives the wireless signal at a reception time. The difference between the reception time and the transmission time is the time it took for the wireless signal to propagate from the signal source to the mobile device. The propagation time can then be related to the distance between the mobile device and the signal source based on the speed of light. If distances between the mobile device and multiple signal sources are determined in this way, the location of the mobile device can be determined by triangulation. This approach, however, is subject to error when the mobile device's time reference is not synchronized with the signal source's time reference.
In an alternative approach, a mobile device can receive a first wireless signal transmitted by a first signal source and a second wireless signal transmitted by a second signal source, with each of the first and second wireless signals including a respective indication of its transmission time. Based on the transmission times and reception times of the respective wireless signals, the mobile device can calculate a difference between the propagation times for the wireless signals transmitted by the first and second signal sources. The propagation time difference is often described as a “Time Difference of Arrival” (TDOA) measurement. Based on the speed of light, the TDOA measurement can be used to define a set of possible locations for the mobile device. Typically, the possible locations lie on hyperbolas having foci corresponding to the locations of the two signal sources used for the TDOA measurement. The mobile device may also obtain one or more additional TDOA measurements, based on wireless signals transmitted by one or more additional pairs of signal sources, to define one or more additional sets of possible locations for the mobile device. The location of the mobile device may then be calculated based on where the sets of possible locations (e.g., hyperbolas) defined by the TDOA measurements intersect.
The TDOA approach has an advantage in that it reduces or eliminates mobile-device time synchronization errors associated with the mobile device using a time reference that is not synchronized with that of the signal sources. For example, by calculating a propagation time difference, the mobile-device time synchronization errors in the individual propagation times from the signal sources can cancel out. Nonetheless, the calculated propagation time difference can be subject to signal-source time synchronization error if the two signal sources are not synchronized to the same time reference. Thus, signal sources used for TDOA measurements are typically synchronized to the same time reference, such as a time reference based on GPS signals, in order to avoid signal-source time synchronization errors.